Power Plane and Decoupling Optimization · Package decoupling modeled usi it / ESR ESLing a...

Power Plane and D liDecoupling

OptimizationpIsaac Waldron

OverviewOverview

Frequency- and time-domain powerFrequency and time domain power distribution system specificationsDecoupling design exampleDecoupling design example

Bare boardAdd d itAdded capacitorsBuried Capacitance

Conclusion

Frequency Domain PDS TargetsFrequency Domain PDS TargetsExcessive impedance seen by a device drawing power f PDS ill l flfrom a PDS will cause power voltage to fluctuateOn a board, impedance must be below target from DC to several hundred MHzWorking in the frequency domain allows quick estimation of power quality

Mag. of Z

|Z|targetZ

PDS Components

Mag. of Z

|Z|targetZ

1MHz1KHz

Switching Electrolytic High Power/GroundPlanes

1 GHz100MHz

Switching Power Supply

Electrolytic Bulk Capacitors

gFrequency Ceramic Capacitors

PlanesBuried Capacitance

Time Domain PDS TargetsTime Domain PDS Targets

S-parameters and impedance areS parameters and impedance are calculated in the frequency domainDevice specifications are typically given inDevice specifications are typically given in the time domainE l i VCC i 10%Example: maximum VCC excursion 10% of nominal value

1.8 V VCC has an allowable range of 1.62 V to 1.98 V

PDS Design FlowPDS Design Flow

EM extraction of impedance

Determine frequencies of

Simulate in time domain to

Choose capacitor or geometric

for critical devices

specification violations

check for compliance

change to address violations

Alter design according to g

findings

Board Imported from LayoutBoard Imported from Layout

Measuring impedance at the six VCC pins on U41

Defining the Target ImpedanceDefining the Target Impedance

To define the target

To define the target impedance we need to consider two VRM

factors:Peak current

Package Model

DC=VCC

pullup

pulldown

logic_in

enableout_of_in

Determines maximum impedance

Spectral powerDetermines cutoff frequency

Driver

Peak CurrentPeak current 37.87

Peak CurrentPeak current 37.87 mASix drivers and 0.18 V 40.00

Ansoft Corporation DriverDriver CurrentCurve Info max

mag(Ipositive(vrm))Transient 37.8706

maximum voltage swing:

a s e t

15.00mag

( ) Ω= m 800mA 87.376V 18.0

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00Time [ns]

Driver Spectrum1.00E-003

Ansoft Corporation DriverXY Plot 4Curve Info

Driver Spectrum

46 11 dB @ 400 MH

95% of driver power is below 667 MHz

1.00E-005

1.00E-004

mag(vrm_pow er)Transient-46.11 dB @ 400 MHz

-51.59 dB @ 83 MHz

1.00E-007

1.00E-006

1.00E-009

1.00E-008

1.00E-012

1.00E-011

1.00E-010

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00Spectrum [GHz]

1.00E 012

findings

Bare BoardBare Board

Target impedance 800

Bare Board

impedance 800 mOhm to 667 MHz

Simulate in time domain

to check for compliance

findings

Time Domain SchematicTime Domain Schematic

pullup

logic_in

Driver

VCC_U41-21VCC_U41-42

VCC_U41-44VCC_U41-63VCC_U41-84

VCC_U41-2

V35pulldown

enable out_of_in

C 800 Mbps data rate

DDR2 IBIS driver into idealVRM

DC=VCC0

DDR2 IBIS driver into ideal termination used as load for PDS

Package decoupling modeled i it / ESR ESLusing a capacitor w/ ESR, ESL

Switching Power NoiseSwitching Power Noise2.40

Ansoft Corporation BareU41 PowerCurve Info pk2pk

V(VCC_U41-2)Transient 0.5427

Shaded area represents time domain specification 1.8 V ± 10% 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00

Time [ns]

Spectral AnalysisSpectral Analysis2.40

Ansoft Corporation BareU41 PowerCurve Info pk2pk

V(VCC_U41-2)Transient 0.5427~11-12 ns period

11-12 ns period corresponds to frequency of 80-90 MHz

~11-12 ns periodThis is confirmed by the spectral plot and correlates with the ideal driver simulation shown earlier

12.50Ansoft Corporation BareSpectral

Curve Info

dB(V(VCC_U41-2))Transient

dB(V(VCC_U41-21))T i t

Name X Y

m1 90.0000 -20.5597

m2 400.0000 -28.4996-13.96 dB @ 90 MHz

d e s u a o s o ea e(peak @ 83 MHz)

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00Time [ns]

-37.50

-27.50

-17.50

Transient

-77.50

-67.50

-57.50

-47.50

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00Spectrum [MHz]

-87.50

h ddfor critical devices

findings

Adding Bulk CapacitorsAdding Bulk CapacitorsAdded two 47 uF capacitors ascapacitors as specified by VRM manufacturer

findings

Bare Board vs. Bulk CapacitorsBare Board vs. Bulk Capacitors

Resonance @ 50 MHz Bare BoardBoard w/

Target impedance 800 Bulk Capsimpedance 800 mOhm to 667 MHz

Bulk CapacitorsBulk Capacitors

Board w/ Bulk CapsTarget

impedance 800Resonance @ 50 MHz

findings

Ansoft Corporation BulkU41 PowerCurve Info pk2pk

11 12 i d

~11-12 ns period

Time [ns]

Spectral AnalysisSpectral Analysis2.40

Ansoft Corporation BulkU41 PowerCurve Info pk2pk

~11-12 ns periodS 11 12 i d

Transient

Same 11-12 ns period as exhibited by bare board

[V] V(VCC_U41-84)

Transient 0.6858

12.50Ansoft Corporation BulkSpectral

Curve Info

dB(V(VCC U41 42))Name X Y

1 90 0000 13 9600

-13.96 dB @ 90 MHz

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00Time [ns]

-37.50

-27.50

-17.50

m1 90.0000 -13.9600

m2 50.0000 -37.1805-37.18 dB @ 50 MHz

Note that resonance @ 50 MHz

-77.50

-67.50

-57.50

-47.50

Note that resonance @ 50 MHz is not excited due as indicated by low spectral content at that frequency

0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00 1000.00Spectrum [MHz]

-87.50

Resonance at 50 MHzResonance at 50 MHz

findings

Choosing a CapacitorChoosing a CapacitorTo reduce the effect 22 nFTo reduce the effect of a resonance, choose a capacitor

22 nF Capacitor

with a low impedance at the resonant ffrequency

Board w/ Bulk Caps

Added HF CapacitorsAdded HF Capacitors

52 20 nF capacitors were added across52 20 nF capacitors were added across the board to reduce high-frequency impedance and to cancel resonance at 50impedance and to cancel resonance at 50 MHz

findings

Bulk vs. HF Capacitors 1Bulk vs. HF Capacitors 1

Board w/ Bulk Caps

Target impedance 800 Board w/

HF Caps 1

No Resonance @ 50 MHzNo Resonance @ 50 MHz

HF Capacitors 1HF Capacitors 1

Board w/HF Caps 1Exceeds 800 mOhm @ 200 MHzTarget

impedance 800impedance 800 mOhm to 667 MHz

No resonance @ 50 MHzNo resonance @ 50 MHz

findings

Ansoft Corporation HF1U41 PowerCurve Info pk2pk

Time [ns]

findings

Extending Low ImpedanceExtending Low Impedance10 1.2 nF capacitors pwere added across the board to extend minimum highminimum high-frequency impedance1 2 nF capacitor was Added capacitors1.2 nF capacitor was chosen due to low impedance at 200 MHMHz4 of these were located near U41located near U41

findings

HF 1 vs. HF 2HF 1 vs. HF 2

New resonance @ 80 MHz

Board w/HF Caps 1

Target impedance 800 e eso a ce @ 80

Board w/HF Caps 2

Impedance exceeds 800 Oh @ 350 MH800 mOhm @ 350 MHz

HF 2HF 2

New resonance @ 80 MHz

Board w/HF Caps 2Target

impedance 800 e eso a ce @ 80impedance 800 mOhm to 667 MHz

Impedance exceeds 800 Oh @ 350 MH800 mOhm @ 350 MHz

findings

Ansoft Corporation HF2U41 PowerCurve Info pk2pk

Time [ns]

Resonance at 80 MHzResonance at 80 MHz

findings

Removing a ResonanceRemoving a ResonanceSix 8 nF capacitorsSix 8 nF capacitors were added near U41 to cancel resonance at 80 MHz

Added capacitors

Choosing a CapacitorChoosing a CapacitorTo reduce the effect 8 2 nFTo reduce the effect of a resonance, choose a capacitor

8.2 nF Capacitor

with a low impedance at the resonant ffrequency

Board w/HF Caps 2

findings

HF 2 vs. HF 3HF 2 vs. HF 3

Board w/HF Caps 2

HF Caps 3

Impedance crosses 800 Oh @ 500

No resonance @ 80 MHz800 mOhm @ 500 MHz

HF 3HF 3

Board w/HF Caps 3Target

impedance 800impedance 800 mOhm to 667 MHz

findings

Switching Power Noise2.40

Ansoft Corporation FinalU41 PowerCurve Info pk2pk

Switching Power Noise

Maximum peak to peak noise of 371 mV

Time [ns]

findings

Buried CapacitanceBuried CapacitanceDue to parasitic i d t it ill binductance it will be impossible to further decouple the board with capacitorscapacitorsUsing a thinner dielectric layer between power and ground planes introducesground planes introduces additional capacitance and reduces high frequency impedanceq y p

dAC ε=Capacitance of

parallel plates: dparallel plates:

findings

HF 3 vs. Buried CapacitanceHF 3 vs. Buried Capacitance

Board w/HF Caps 3B d /

Buried Capacitance

Impedance crosses 800 Oh @ 667800 mOhm @ >667 MHz

Buried CapacitanceBuried Capacitance

Board w/Buried Capacitance

Target impedance 800

Target impedance specification metCapacitanceimpedance 800

mOhm to 667 MHz

findings

Switching Power Noise2.40

Ansoft Corporation BuriedU41 PowerCurve Info pk2pk

Switching Power Noise

Maximum peak to peak noise 273 mV24% smaller than limit

24% smaller than limit

Time-domain noise specification met

Shaded area represents time domain specification 1.8 V ± 10%

Time domain noise specification met

0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 80.00 90.00 100.00Time [ns]

ConclusionConclusionAnsoft software allows PCB engineers to gdesign effective decoupling solutions for their PCBsImpedance and resonant mode simulationsImpedance and resonant mode simulations connect the frequency domain to the spatial domain and allow selection of capacitor value pand placementFrequency domain extractions are useful for quickly optimizing PDS designs but timequickly optimizing PDS designs, but time domain simulations are necessary to ensure compliance with device specs

Power Plane and Decoupling Optimization · Package decoupling modeled usi it / ESR ESLing a...

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